Conventional filter manufacturing machines include a storage vessel storing, for example, acetate tow for use as filter fibers, and the tow (filter fibers) is fed from the storage vessel along a predetermined feed path. In the process of feeding, the tow is bloomed. Subsequently, a plasticizer such as triacetin is added to the tow, and the tow is supplied to a rod forming device. In the rod forming device, the tow is formed into a rod, and the rod-shaped tow is wrapped in paper to continuously form a filter rod.
The tow is formed into a rod by means of a section called forming tube. While passing through the forming tube, the tow is compressed and formed into a rod. There has also been known a technique whereby the tow is formed into a rod and at the same time a through hole is formed in the rod to obtain a hollow cylindrical rod while the tow passes through the forming tube.
In order to make such a through hole in the rod, a mandrel is previously arranged inside the forming tube in alignment with the axis of the forming tube. The tow introduced into the forming tube is formed into a rod around the mandrel, continuously delivered, and pulled out of the mandrel, whereupon the rod with a through hole is obtained. It is, however, often the case that the tow projects inward from the surface defining the through hole, and a filter rod with such a through hole is regarded as low in quality and rejected as a defective article. Also, if the surface region of the filter rod around the through hole is not satisfactorily high in hardness, the through hole is crushed when the filter rod is cut in a subsequent process, leading to lowering of the quality. In order to prevent crushing of the through hole, the amount of the plasticizer used may be increased to thereby increase the hardness of the filter. If the plasticizer is used in a large amount, however, a problem arises in that the filter itself is dissolved or gives off an offensive smell.
Projection of the tow from the surface defining the through hole is thought to occur for the following reasons. Since the mandrel has an outer shape with the same diameter and size as those of the through hole to be formed, the fiber density of the tow becomes uneven and also the tow is not acted upon by a force that presses the surface of the through hole from inside. Further, the fibers forming the tow are allowed to adhere to each other only at the moment the tow is introduced into the gap between the forming parts, and therefore, adhesion of the tow is not high enough. The consequent projection of the tow from the surface defining the through hole spoils the appearance of filters obtained.
On the other hand, Patent Document 1 discloses an apparatus for manufacturing a hollow cylindrical filter rod. In this manufacturing apparatus, a tube is arranged in alignment with the axis of a filter rod to be formed, and tow is gathered around the tube and hardened to form a hollow filter. The apparatus, however, requires an additional member, namely, the tube, which leads to increase in the number of component parts. Also, a tube feed mechanism needs to be additionally provided, making the apparatus and its control complex.